The present invention relates generally to broadcast and multicast services in mobile communication networks, and more particularly to a hierarchical transmission of a broadcast service with blind retransmission. The 3rd Generation Partnership Project (3GPP) includes a work item known as Long Term Evolution (LTE) to improve the Universal Mobile Telecommunications System (UMTS) standard in response to increased demand for mobile data services. Multimedia Broadcast and Multicast Service (MBMS) is one service offered in the LTE standard. MBMS is a broadcast service similar to conventional television and radio broadcast systems in which the same content is transmitted to multiple users located in a specific service area, referred to herein as the broadcast service area. Mobile TV, a service that enables mobile terminals to receive ordinary television programs, is one example of an MBMS application.
Two different modes are contemplated for providing MBMS: a single cell transmission and an MBMS Single Frequency Network (MBSFN). With single cell transmission, the broadcast content is transmitted to the user from a single cell and a user may be handed over as it moves from one cell to another. In MBSFN, multiple transmitters in different cells transmit the same broadcast service synchronously using the same resources. There is no need for the network to handover the user as the user moves between cells within the MBSFN service area.
The minimum signal to interference plus noise ratio (SINR) in the MBSFN service area is determined by (thermal) noise and/or interference from remote transmitters. The minimum SINR is inversely proportional to the distance between neighboring base stations, referred to herein as Inter Site Distance (ISD). That is, the minimum SINR for the broadcast service area decreases with increasing ISD.
In the case of a homogeneous MBSFN service area where the ISD is roughly the same between any pair of neighboring base stations, the minimum SINR, denoted SINRmin, can be determined from the known network parameters and the transmission mode can be chosen accordingly so that the desired reception quality in terms of bit or packet error rate is achieved at the given SINRmin. Typically, the transmission mode will not be adapted according to the absolute minimum SINR in the MBSFN, but according to an agreed low percentile, e.g., the 5% value, of the SINR distribution. This value will nevertheless be denoted SINRmin in the following description. In general, a higher SINRmin equates with higher throughput.
In practical networks, the ISD will not be uniform. Some areas may have a relatively large ISD while others have a relatively small ISD. Furthermore, the propagation conditions will be different in different cells, depending on the building structure, antenna heights, percentage of indoor users, and other factors. Therefore, determining an appropriate transmission mode for all cells in the MBSFN service area can be problematic. In existing broadcasting networks, e.g., DVB-T, the transmission mode is simply set according to the SINRmin occurring in the MBSFN. This SINRmin will typically occur in the geographical areas that are covered with sites of large ISD.
In network deployments/configurations where the SINRmin is limited by noise, the SINRmin can be increased by increasing the transmit power of the base stations closest to the location where SINRmin occurs. This solution is however, limited by the maximum transmit power of the transmitter and probably by regulatory requirements. In other deployments/configurations where the SINRmin is determined by the interference from remote base stations, increasing the transmit power bears the risk of causing increased interference in other locations in the MBSFN service area, thereby only moving the location where the SINRmin occurs, without actually increasing SINRmin.